The Recent Researches in Solar PV Panel Busbars and Developments

Solar Panel – Understanding Busbars

Introduction

Since working on the improvement of existing solar cell technology is becoming increasingly harder; researchers are struggling to squeeze out more from conventional cells and an increasing trend in the industry is to re-think cell design and manufacturing to further increase efficiency and reliability. The approaches are manifold and not only geared to cell performance and reliability but also significantly towards the reduction of material costs.

What are Busbars?

Silicon solar cells are metallized with thin strips printed on the front and rear of a solar cell. These front and rear contact strips are referred to as busbars. The purpose of busbars in solar cells is to conduct the DC power generated by the cell when photons hit the cells. These contacts are printed onto the surface via a technology called screen printing. Usually, solar cell busbars are made of a silver paste or similar high conductivity materials. The silver plating is necessary to improve current conductivity (front side) as well as to reduce oxidization (rear side). Perpendicular to the busbars is the metallic and super-thin grid “fingers”, which are connected by the busbar. Basically, the fingers collect the generated DC current and deliver it to the busbars. During the module manufacturing process, String and finger wires are soldered to the busbars to connect solar cells in a combination of series and parallel. The cluster of strings is then connected to the solar junction box. Crucial parameters are in this regard are the height and width of the busbars, the width of the fingers, spaces between the fingers and the busbars as well as the metal type and quality. As individual solar junctions produce low voltage and to obtain suitable voltages, the solar cells must be connected in series, forming rows.

Why are Busbars required?

The older practice was to have solar cells with typically two busbars. However, as the technology evolved and the industry moved towards higher efficiencies, the busbars in most solar cells were increased to 2BB, 3BB and now to 5BB. This increased number of busbars reduces the internal resistance losses, which is due to the lesser distance between the busbars. They have also helped in increasing the cell efficiency and hence ultimately improving the module efficiency. Even though the higher quantity of busbars increases the shading of the solar cell, yet the overall performance of multi busbar cells is much better than that of conventional cells. This is because of aspects such as reduction of the effective finger length between the busbars, which reduces finger resistance losses. As a matter of fact, finger wire and busbar for solar modules are generically the same. The busbars must carry the bus current, which is many times larger in size than the finger current. Hence the busbars must be thicker and broader.

Recent Trends

Multi busbar cells

Multi busbar cells, noticeably five busbar cells, are currently one of the major trends in solar cell and module design. As micro cracks typically occur between busbars, the impact of such cracks is thus reduced towards smaller affected cell slices between two busbars. Thus, as compared to conventional cells, the long-term reliability of a multi busbar cell even in the event of micro cracks is higher.

Busbar-less cells

Contrary to the multi busbar approach, other companies are going an entire busbar-less way.

The advantages of busbar-less cells are

• much better power production-to-space ratio

• more flexible module size design – conventional module design is limited by cell size and spacing requirements significantly reduced shading less potential cell-inherent defects such as micro cracks, often caused by cell soldering

• saving of busbar material costs.

While the multi busbar and busbar-less designs are focusing on increasing performance and reliability, other approaches focus on the reduction of material costs, primarily by minimizing the cost-driving silver metallization of the busbars as with the dash-line busbar designs by entirely replacing silver for the solar cell metallization with alternative materials such as tin or nickel.

Dash-line pattern busbars

The dash-line pattern in recent years a more cost-efficient alternative to the traditional full line busbars evolved called dash-line pattern busbars, reducing the usage of the expensive silver paste. Various types of dash-line busbars exist, such 3-dash, 5-dash, 6-dash, and even 8-dash busbars. As cost reductions often go hand in hand with long-term quality and reliability reductions, this can also be observed for the dash-pattern busbar inventions.

Finger wire optimization

When optimizing the performance of solar cells, the finger wires also offer room for improvements. One such improvement involves the usage of round finger wires, which – in comparison to the rectangular-shaped wires – come with less shading due to their reduced space.

Busbar-less solar cells

Contrary to the multi busbar concept, some manufacturers are going the entire busbar-less way; they use overlapping solar photovoltaic cell segments that are directly electrically connected with each other.

The advantages of busbar-less cells are apparent:

• Reduce inactive space between solar cells.

• More flexible module size design

• Significant reduction of power loss caused by shading

• Fewer microcracks, often due to cell soldering

• Saving of busbar material costs

Conclusion

This article is focused on understanding what sets different types of solar panel busbars and more importantly, how the number of busbars affects your solar module’s production efficiency. Going ahead it is expected to see more and better busbar designs, especially now that the solar technology is moving towards increasing the efficiency of PV modules while reducing the overall cost.